The present invention relates to the nondestructive testing of structural materials, work pieces, components or the like made of an electrically conductive material whereby the test includes passing a movable electromagnetic system across the piece to be tested which system is comprised of at least one pole piece being surrounded by a magnetizing coil and being on one side connected to a soft magnetic (high permeability with little or no hysteresis) return path. Moreover, the equipment is assumed to provide for ultrasonic transmission and receiving.
Electrodynamic ultrasonic transducers are known and have been described extensively in the literature as to both their structure as well as to their function. A significant number of papers has been devoted to analysis and description of such transducers. Basically they are comprised in each instance of a transmission coil and of a receiver coil or of a combined time sharing transmission/receiving coil. They are arranged in a static electromagnetic field and particularly physical relation to the piece to be tested and here particularly close to the surface thereof. Upon electrically energizing the transducer (transmitter or transmitting) coil an ultrasonic pulse is produced in the work piece. Either the same coil or the receiving coil will receive any response by the work piece such as an echo and produces an electrical pulse (physical movement in the static magnetic field) having a particular temporal amplitude possibly also frequency relation to the initial pulse.
All these ultrasonic transducers as described are to be used on planar as well as curved work pieces for purpose of nondestructive testing; there is no basic limitation involved. As an example see e.g. European Pat. No. 24 707. This equipment, however, exhibits a certain restriction in that preferably only one or two coils at the most can be used. This limitation results from the fact that the conversion of ultrasonic into electromagnetic energy and vice versa requires a relatively high static magnetic fields, and for this reason only a limited amount of space is available for effective transduction unless special steps are taken for limiting e.g. any spreading effect of that field. In order to improve and enlarge the testing power and capability, i.e. in order to speed up the overall testing process it has been suggested to provide a plurality of such ultrasonic transducers and in a rather close spacial relationship. Also they are associated with a common static magnetic field. In order to avoid interference these transducers are controlled to operate in a certain sequence under utilization of suitably programmed electronic circuit. In other words, one uses the high speed operation of the transducer and pulse transmission, propagation and receiving to bridge the relatively slow movement between transducer and workpiece to be tested.
Also known is a transmitter arrangement (see German printed patent application No. 28 45 579) wherein the transmitter is associated with one pole and several receiver coils are associated with the same or the other piece. The test speed attainable with such a device is still not regarded as sufficient since the test speed is limited by the size of the magnet even if one uses more and more transducer coils or coil pairs within a homogenic magnetic field.